1. Field of the Invention
The instant disclosure relates to a high voltage constant current buffer driver circuit; in particular, to a buffer circuit that can provide a stable output current even when operating at different supply voltages that are within a wide voltage range.
2. Description of Related Art
A high voltage constant current buffer driver circuit is often used in an Intelligent Power Module (IPM) to provide a sufficient driving power to drive a NPN bipolar junction transistor (BJT). Thus, the current provided by the buffer circuit to drive the BJT cannot be too large. If the base current provided by the buffer circuit to drive the BJT is too large, the BJT will be over driven and the storage time of the BJT will be increased, and thereby the propagation delay of the output of the Intelligent Power Module (IPM) will be increased.
Referring to FIG. 1, FIG. 1 shows a circuit diagram of a conventional constant current buffer circuit. As shown in FIG. 1, to be able to operate at a supply voltage that may vary within a wide voltage range, there are an NMOS transistor MN1′ and a PMOS current mirror comprising PMOS transistors MP1′ and MP2′, configured in the conventional constant current buffer circuit 100. Specifically, the NMOS transistor MN1′ and the PMOS transistor MP1′ need to be high-voltage transistors. The PMOS transistor MP2′ is configured as a diode connected transistor to clamp the voltage at the output of the level shifter LS and the gate of the PMOS transistor MP1′ to a predetermined voltage. The source of the PMOS transistor MP1′ is connected to the supply voltage of the buffer circuit 100 to receive the supply voltage. The gate and drain of the diode connected PMOS transistor MP2′ are connected together, and they are further connected to the gate of the PMOS transistor MP1′.
Even though in this manner the buffer circuit 100 can operate at a supply voltage that may vary within a wide voltage range, the output current of the buffer circuit 100 will also vary with the changes of the supply voltage. Especially, the output current of the buffer circuit 100 will dramatically increase when the supply voltage of the buffer circuit 100 increases. Thus, if the buffer circuit 100 is used in an Intelligent Power Module (IPM), the NPN BJT power transistor which is a load to the buffer may be over driven by a large output current and thus there will be an increase in propagation delay of the output of the Intelligent Power Module (IPM).